This invention relates to a process for the electrochemical reduction of terephthalic acid to p-hydroxymethylbenzoic acid (pHMBA) and more particularly to improvements in the electrical efficiency of the said reaction and to operation by treatment of the cathode to obtain a mercury overlay as an amalgam on the cathode surface and the addition of a soluble salt of mercury, which regenerates the amalgam overlay, to the electrolysis catholyte solvent.
Inasmuch as the cost of electric power utilization in the electrochemical conversion of terephthalic acid to p-hydroxymethylbenzoic acid has a direct bearing on the economics of this process it is highly desirable to increase current efficiency and to hold side reactions which do not lead to the desired product to a minimum. If only one series of reactions were taking place in the instant process, that is, the electrochemical reduction of terephthalic acid to p-hydroxymethylbenzoic acid at the cathode, the ideal of 100% conversion of the starting material to the desired product would be achieved. However, as in the case with most electrochemical reactions, this particular reaction is not so simple and many competing reactions can take place in the electrolysis cell. The resulting presence of 4-carboxybenzaldehyde (4-CBA), dihydroxymethylbenzene, toluic acid and other impurities render resulting p-hydroxymethylbenzoic acid undesirable for use as a monomer for polymer applications without further expensive purification.
It is well-known that in the cathodic reduction of carboxylic acids that two types of products can result, either the corresponding aldehyde in a two-electron process or the hydroxymethyl compound in a four-electron process wherein the aldehyde is further reduced to the alcohol. (M. Baizer, Organic Electrochemistry, Dekker, N.Y., (1973), 414). The alcohol can be further reduced to the methyl group.
A further complication in the electrolysis of terephthalic acid to p-hydroxymethylbenzoic acid is the development, as the electrolysis proceeds, of a deactivating or poisoning layer which accumulates on the cathode if a solid cathode such as lead is used. Current efficiency accordingly suffers. Continuous mercury cathode cells have been developed for the electrolytic reduction of phthalic acid to overcome this same problem (P. C. Condit, IEC, 48, 1252 (1956)). However, use of solid cathodes versus the use of liquid cathodes has the advantage of simplicity in construction and versatility.
In the prior art, it is known that the reduction of aromatic carboxylic acids at lead or mercury cathodes in a protic solvent (proton-donor) i.e., alcoholic, etc., gives excellent yields of corresponding benzyl alcohols (Chem. Ber., 38, 1747 (1905); ibid, 39, 2933 (1906); Ann., 417, 69 (1929); Org. Syn. 21, 10 (1941)). Baizer (op. cit. 417) suggests that the mechanism of this process in strongly acidic alcoholic media may be that the carboxylic acid is reduced in its protonated form, RCOOH.sub.2.sup.+, or that the acid first forms an ester with the alcoholic solvent, the ester being more reducible than the acid. Ono, Nippon Kagaku Zasshi, 75, 1195-9 (1954) (CA51:12704b) teaches the electrolytic reduction of phthalic and isophthalic acid and their esters using a mercury cathode gave two types of reactions, reduction of the side chain and the benzene ring, phthalic acid giving dihydrophthalic acid and dimethyl isophthalate giving m-hydroxymethylbenzoic acid. Ono, et al., J. Chem. Soc., Japan, Pure Chem. Section, 74, 907-11 (1953) (CA48:8082d) reported electrolytic reduction of dimethyl terephthalate to p-methylhydroxymethylbenzoate using a mercury cathode. German Offenlegungsschrift No. 24 28 878 teaches a process for production of p-hydroxymethylbenzoic acid esters by electrochemical reduction of dimethylterephthalate on solid electrode cathodes (lead, zinc, cadmium, graphite and amalgamated metals--lead, copper, etc.) with methanol as solvent. However, the poisoning effect has plagued the application of electrolytic reduction processes to aromatic carboxylic acids. Natarajan, et al., Electrochem. Technol., 2 (5-6), 151-6 (1964) (CA61:6940c) reports in the electrolytic reduction of benzoic acid to benzyl alcohol using a rotating lead cathode, the reaction proceeds with good current efficiency until the electrolyte is saturated with benzyl alcohol, at which point the cathode becomes covered with a layer of benzyl alcohol, effectively hindering the diffusion of benzyl alcohol with resultant decrease in current efficiency. German Offenlegungsschrift No. 26 42 496 teaches a process for the production of p-hydroxymethylbenzoic acid by the electrochemical reduction of terephthalic acid in the presence of ammonia (a basic protic solvent) using electrodes such as mercury, lead, cadmium and antimony. Lead is indicated as especially suitable. Unfortunately, activity drops off rapidly after a few minutes. The problem of maintaining cathode activity is met by discontinuing the direct current periodically and shorting out the cell for periods of 1/2 to 3 minutes. From the physical constants given in this publication for the p-hydroxymethylbenzoic acid (melting point of 182.5.degree. to 183.5.degree. C.; conversion 100% and selectivity 91%) it is probable that the electrochemical product of terephthalic acid is not a pure product but contains 4-carboxybenzaldehyde or toluic acid. The large excess of current taught as used, 15.3 Faradays, 4.0 Faradays being 100% of theoretical, indicates the difficulty of maintaining cathode activity of a lead cathode.
Accordingly, it is an object of the present invention to develop an electrochemical process for the manufacture of p-hydroxymethylbenzoic acid from terephthalic acid which avoids the above disadvantages. It is an object of the present invention to provide a process for production of p-hydroxymethylbenzoic acid wherein the production of by-product impurities, namely, 4-carboxybenzaldehyde, dihydroxymethylbenzene and toluic acid is minimized. It is a further object of this invention to increase the current efficiency of the electrochemical reduction process over that of previously known methods for electrochemical reduction of terephthalic acid to p-hydroxymethylbenzoic acid. It is another object of this invention to provide an efficient continuous process for the electrochemical production of p-hydroxymethylbenzoic acid wherein discontinuance of the reductive process is not required by the development of a poisoning barrier upon the cathode. Other objects will appear hereinafter.